The influence of aerosol particle size and route of administration of M. pneumoniae on the course of experimental infection was studied in golden Syrian hamsters. In contrast to large-particle aerosols, which were deposited in the upper respiratory tract (URT), small-particle aerosols were distributed throughout the entire respiratory tract and were more effective in initiating infection and conferring protection following challenge. In other studies, hamsters inoculated orally with either wild-type or ts 640 M. pneumoniae underwent a silent URT infection and were significantly resistant to subsequent intranasal challenge. In an effort to establish an experimental disease with a closer resemblance to human disease 3 rhesus monkeys were inoculated via the URT with l0 CFU. Each animal underwent an asymptomatic infection with a serological response and shedding of the organism. Inoculation of l0 CFU in the URT and LRT of 3 squirrel monkeys also resulted in a silent infection. Subsequent challenge of these monkeys was followed by a shorter, more restricted infection without accompanying disease. There was no evidence of transmission of the organism to broth-inoculated cagemate controls.